A warm ocean current that splits fish populations in two. Rockfish on either side became genetically distinct when the Japan Sea was isolated 1.5 million years ago.
Image Science and Analysis Laboratory, NASA-Johnson Space Center. "The Gateway to Astronaut Photography of Earth.", Public domain, via Wikimedia Commons
A current running north
The Tsushima Strait, the body of water separating Japan from the Korean Peninsula, is a geographic feature. It is a powerful engine of evolution, driven by a relentless northbound current. This channel is dominated by the Tsushima Warm Current, a major branch of the warm Kuroshio Current. The Tsushima Current flows from the East China Sea into the Sea of Japan, carrying a massive volume of water—its transport averages 2.64 million cubic meters per second. This constant, powerful flow acts as a one-way street for marine life, making it extremely difficult for the larvae of coastal species to travel south against it.
This effective barrier has created a distinct genetic separation between populations of the same species on either side of the strait. Many marine organisms show a clear phylogeographic pattern, with a "Sea of Japan lineage" and a "Pacific Ocean lineage". The most well-studied example is the black rockfish, Sebastes inermis. Populations of this slow-moving, coastal fish in the Sea of Japan are genetically distinct from those living on the Pacific side of the Japanese archipelago. The current effectively prevents gene flow between the two groups, pushing them down separate evolutionary paths.
Deep-time isolation
The story of this species divide is about the modern current, and the deep geological history of the region. During the repeated ice ages of the Pleistocene epoch, global sea levels fell dramatically. The straits connecting the Sea of Japan to the wider ocean—the Tsushima, Tsugaru, and Soya—are all relatively shallow. As sea levels dropped by as much as 140 meters, these straits became dry land, completely isolating the Sea of Japan.
This isolation turned the sea into a cold and brackish body of water for long periods. Many marine species within the Sea of Japan went extinct during these glacial periods. Coastal populations on the Pacific side, however, survived in what scientists call a refugium. When the ice caps melted and the seas rose again, the straits reopened. Species from the Pacific refugium recolonized the Sea of Japan. But in the time they were separated, the populations had accumulated enough genetic differences to be considered distinct lineages. This process of separation and reconnection happened multiple times, reinforcing the genetic divide. The divergence for some species, like the crangonid shrimp Argis lar, began as early as one million years ago.
The Tsushima Strait is a living laboratory of phylogeography, showing how ocean currents and ancient geological events shape the distribution and evolution of life in the sea. The genetic split is not limited to rockfish; similar patterns are found in species of snails, seaweed, and clams.
💡Fun Facts
The *Sebastes inermis* rockfish is part of a species complex that includes two other closely related rockfish, *S. cheni* and *S. ventricosus*, which live in the same habitats and are distinguished by color.
The Tsushima Warm Current is so significant that it influences the migration of large fish like Pacific bluefin tuna and yellowtail, which travel north along the current to feed.
Rockfish are known for their extremely long lifespans; some species can live for over 140 years.
During the last glacial maximum, the isolation of the Sea of Japan led to severe anoxic (low-oxygen) conditions in its deep waters.
The strait is a major body of water. It can be viewed from numerous coastal points on Kyushu island, the smaller Tsushima Island, and the southwestern coast of Honshu. Terrain varies from sandy beaches to rocky, cliff-lined coasts.
